1. Field of the Invention
The present invention relates to control methods and circuits for LED chains, and more particularly to control methods for short protection in LED chains.
2. Description of the Prior Art
In an age concerned with energy conservation and carbon reduction, light-emitting diodes (LEDs) are already a widely adopted light source due to their superior lighting efficiency and miniature component size. For example, LEDs have already replaced cold-cathode fluorescent lamps (CCFLs) as a backlight in current liquid crystal display (LCD) panels.
FIG. 1 is a diagram illustrating an LED power supply 18 used in a backlight module of an LCD panel, which is primarily used to control lighting of LED chains L1-LN. Each LED chain has a plurality of series-connected LEDs. Backlight controller 20 controls a power switch of booster 19 to cause an inductive element to draw energy from input node IN, and release energy into output node OUT, so as to establish an appropriate output voltage VOUT on output node OUT to drive the LED chains. Backlight controller 20 detects output voltage VOUT through over-voltage protection node OVP and voltage divider resistors RD1, RD2.
Driving nodes LED1-LED4 of backlight controller 20 are connected to LED chains L1-L4, respectively, for draining driving current of LED chains L1-L4, and controlling current flowing through each LED chain to be approximately equal to achieve the goal of uniform brightness.
Backlight controller 20 may also determine whether any LED encounters a fault condition from driving nodes LED1-LED4, so as to trigger related protection. For example, if LED detection voltage VLED-1 on driving node LED1 is continually 0V, LED chain L1 may be an open-circuited LED chain, where at least one LED thereof is open-circuited, in which case backlight controller 20 turns off driving of LED chain L1. This type of protection is typically called open circuit protection. In another example, if LED detection voltage VLED-1 on driving node LED1 is much greater than LED detection voltage VLED-2 on driving node LED2, it can roughly be ascertained that LED chain L1 has a few LEDs that are short-circuited, and driving of LED chain L1 can be turned off. This type of protection is typically called short circuit protection.
However, open circuit protection and short circuit protection may interfere with each other. Thus, an appropriate process is needed to activate or stop open and short circuit protections, so as to achieve the desired protection effect.